Method of synthesizing silicoaluminophosphate molecular sieves

ABSTRACT

In a method of synthesizing a silicoaluminophosphate molecular sieve, a synthesis mixture is prepared by combining a source of phosphorus and at least one organic directing agent; and then introducing a source of aluminum into the combination of the phosphorus source and organic directing agent, wherein the temperature of the combination is less than or equal to 50° C. when addition of the source of aluminum begins. After addition of a source of silicon, the synthesis mixture is heated to a crystallization temperature of between about 100° C. and about 300° C. and the molecular sieve is recovered.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/985,496 filed Nov. 10, 2004, now U.S. Pat. No.7,090,814, the entire contents of which is incorporated herein byreference.

FIELD OF INVENTION

This invention relates to a method of synthesizingsilicoaluminophosphate molecular sieves and to the use of the resultantmolecular sieves as catalysts for the conversion of oxygenates,particularly methanol, to olefins, particularly ethylene and propylene.

BACKGROUND OF INVENTION

Light olefins, such as ethylene, propylene, butylenes and mixturesthereof, serve as feeds for the production of numerous importantchemicals and polymers. Typically, C₂-C₄ light olefins are produced bycracking petroleum refinery streams, such as C₃+ paraffinic feeds. Inview of limited supply of competitive petroleum feeds, production of lowcost light olefins from petroleum feeds is subject to waning supplies.Efforts to develop light olefin production technologies based onalternative feeds have therefore increased.

An important type of alternative feed for the production of lightolefins is oxygenates, such as C₁-C₄ alkanols, especially methanol andethanol; C₂-C₄ dialkyl ethers, especially dimethyl ether (DME), methylethyl ether and diethyl ether; dimethyl carbonate and methyl formate,and mixtures thereof. Many of these oxygenates may be produced fromalternative sources by fermentation, or from synthesis gas derived fromnatural gas, petroleum liquids, carbonaceous materials, including coal,recycled plastic, municipal waste, or any organic material. Because ofthe wide variety of sources, alcohol, alcohol derivatives, and otheroxygenates have promise as an economical, non-petroleum sources forlight olefin production.

The preferred process for converting an oxygenate feedstock, such asmethanol, into one or more olefin(s), primarily ethylene and/orpropylene, involves contacting the feedstock with a crystallinemolecular sieve catalyst composition. Among the molecular sieves thathave been investigated for use as oxygenate conversion catalysts, smallpore silicoaluminophosphates (SAPOs), such as SAPO-34 and SAPO-18, haveshown particular promise. SAPO-34 belongs to the family of molecularsieves having the framework type of the zeolitic mineral chabazite(CHA), whereas SAPO-18 belongs to the family of molecular sieves havingthe AEI framework type. In addition to regular orderedsilicoaluminophosphate molecular sieves, such as the AEI and CHAframework types, disordered structures, such as planar intergrowthscontaining both AEI and CHA framework type materials, are known and haveshown activity as oxygenate conversion catalysts.

The synthesis of silicoaluminophosphate molecular sieves involvespreparing a mixture comprising a source of water, a source of silicon, asource of aluminum, a source of phosphorus and at least one organicdirecting agent for directing the formation of said molecular sieve. Theresultant mixture is then heated, normally with agitation, to a suitablecrystallization temperature, typically between about 100° C. and about300° C., and then held at this temperature for a sufficient time,typically between about 1 hour and 20 days, for crystallization of thedesired molecular sieve to occur.

As is the case with the production of most synthetic molecular sieves,the synthesis of silicoaluminophosphates, and in particular intergrownforms thereof, must be closely controlled in order to avoid or minimizethe production of impurity phases that can adversely affect thecatalytic properties of the desired product.

According to the present invention, it has now been found that the orderof addition of the starting materials, particularly of the aluminumsource, and more particularly of both the aluminum source and thesilicon source, and the temperature of the synthesis mixture during theaddition of the starting materials can significantly impact the successof SAPO synthesis processes, particularly when conducted on a large,commercial scale. Thus, the organic directing agent used in thesynthesis of silicoaluminophosphates is often a basic compound, such asa basic nitrogen compound, whereas an attractive phosphorus source isphosphoric acid or a similar phosphorus acid. The mixing of thesematerials can therefore generate heat and hence raise the temperature ofthe synthesis mixture. It has now been found that such a rise intemperature can lead to undesirable side reactions and possibleproduction of impurity phases if the aluminum source is present in themixture or is added thereto before the mixture has been allowed to cool.

U.S. Pat. No. 5,879,655 discloses a method for preparing a crystallinealuminophosphate or silicoaluminophosphate molecular sieve and teachesthat it is critical, especially in large scale preparations, that atleast some of the organic directing agent be added to the aqueousreaction mixture before a significant amount of the aluminum source isadded, since otherwise the aluminum can precipitate to produce a viscousgel. In particular, the addition of the phosphorus source, aluminumsource, and the organic directing agent to the aqueous reaction mixtureis controlled so that the directing agent to the phosphorus molar ratiois greater than about 0.05 before the aluminum to phosphorus molar ratioreaches about 0.5.

According to Example 1 of U.S. Pat. No. 5,879,655, SAPO-11 can beproduced from a synthesis mixture obtained by initially adding 17.82 kgof 86% H₃PO₄ to 8.59 kg of deionized ice in a stainless steel drum withexternal cold water cooling. 9.70 kg of aluminum isopropoxide and 21.0kg of deionized ice are then added simultaneously in small incrementsover a time period of 1.5 hours with mixing using a standard mixingimpeller and homogenization using a Polytron. 3.49 kg ofdi-n-propylamine are then added slowly with mixing. An additional 21.59kg of aluminum isopropoxide and 18.0 kg ice are added in smallincrements over a time period of four hours with mixing/homogenization,followed by an additional 3.49 kg of di-n-propylamine. 2.30 kg of fumedsilica (Cabosil M-5) are then added with mixing/homogenization until >95weight percent of the particles in the mix are smaller than 64 microns.It is reported that, during the entire procedure, the temperature of themixture never exceeds 30° C.

Example 2 of U.S. Pat. No. 5,663,480 discloses the synthesis of acrystalline titanoaluminophosphate from a mixture obtained by placing34.6 g of phosphoric acid (85% by weight aqueous solution) into a beakerhaving a capacity of 500 ml and then adding 73.6 g of tetraethylammoniumhydroxide (20% by weight aqueous solution). After stirring, theresulting mixture is cooled to room temperature, and to this mixture,18.0 g of ion-exchanged water and 21.9 g of pseudo-boehmite are added,and then 15.8 g of titanium tetraisopropoxide is also added. After thecontents of the beaker are stirred for 2 hours, the resulting mixedsolution is poured into an autoclave to carry out hydrothermalsynthesis.

International Patent Publication No. WO 02/70407, published Sep. 12,2002 and incorporated herein by reference, discloses asilicoaluminophosphate molecular sieve, now designated EMM-2, comprisingat least one intergrown form of molecular sieves having AEI and CHAframework types, wherein said intergrown form has an AEI/CHA ratio offrom about 5/95 to 40/60 as determined by DIFFaX analysis, using thepowder X-ray diffraction pattern of a calcined sample of the molecularsieve. Synthesis of the intergrown material is achieved by mixingreactive sources of silicon, phosphorus and a hydrated aluminum oxide inthe presence of an organic directing agent, particularly atetraethylammonium compound. The resultant mixture is stirred and heatedto a crystallization temperature, preferably from 150° C. to 185° C.,and then maintained at this temperature under stirring for between 2 and150 hours. In the Examples, various sequences are disclosed forproducing the EMM-2 synthesis mixture, in which the phosphorus source isinitially combined with either the directing agent or the silicon sourceand then the remaining components, including the aluminum component, areadded without prior cooling.

U.S. Pat. No. 6,334,994, incorporated herein by reference, discloses asilicoaluminophosphate molecular sieve, referred to as RUW-19, which isalso said to be an AEI/CHA mixed phase composition. DIFFaX analysis ofthe X-ray diffraction pattern of RUW-19 as produced in Examples 1, 2 and3 of U.S. Pat. No. 6,334,994 indicates that these materials arecharacterized by single intergrown forms of AEI and CHA structure typemolecular sieves with AEI/CHA ratios of about 60/40, 65/35 and 70/30.RUW-19 is synthesized by initially mixing an Al-source, particularlyAl-isopropoxide, with water and a P-source, particularly phosphoricacid, and thereafter adding a Si-source, particularly colloidal silicaand an organic template material, particularly tetraethylammoniumhydroxide, to produce a precursor gel. The gel is then put into a steelautoclave and, after an aging period at room temperature, the autoclaveis heated to a maximum temperature between 180° C. and 260° C.,preferably at least 200° C., for at least 1 hour, with the autoclavebeing shaken, stirred or rotated during the entire process of aging andcrystallization. Factors which are said to enhance the production of themixed phase RUW-19 material include maintaining the SiO₂ content of thegel below 5%, reducing the liquid content of the gel after addition ofthe SiO₂ source and crystallization at temperatures of 250° C. to 260°C. Pure AEI and CHA phases are said to be favored at temperatures of200° C. to 230° C.

SUMMARY OF INVENTION

In one aspect, the invention resides in a method of synthesizing asilicoaluminophosphate molecular sieve, the method comprising:

-   -   (a) preparing a synthesis mixture comprising the steps of:        -   (i) combining a source of phosphorus and at least one            organic directing agent;        -   (ii) introducing a source of aluminum into the combination            of the phosphorus source and organic directing agent, the            temperature of said combination being less than or equal to            50° C. when addition of said source of aluminum begins;    -   (b) heating said synthesis mixture to a crystallization        temperature of between about 100° C. and about 300° C.; and    -   (c) recovering said molecular sieve.

Conveniently, the temperature of said combination of the phosphorussource and organic directing agent is less than or equal to 40° C.,preferably less than or equal to 30° C., and more preferably in therange of 10° C. to 30° C., when addition of said source of aluminumbegins.

In one embodiment, the temperature of said combination of the phosphorussource and organic directing agent is controlled so that saidtemperature does not exceed 50° C., preferably does not exceed 40° C.,more preferably does not exceed 30° C., and most preferably is withinthe range of about 10° C. to about 30° C., during substantially theentire step (i). Conveniently, the temperature of said combination iscontrolled by cooling the combination and/or cooling one or both of thephosphorus source and organic directing agent.

Conveniently, the source of aluminum is an inorganic aluminum compound,such as a hydrated aluminum oxide and particularly boehmite orpseudoboehmite.

Conveniently, preparing said synthesis mixture also comprisesintroducing a source of silicon into said combination of the phosphorussource and organic directing agent. The silicon source can be introducedinto said combination before, after or simultaneously with theintroduction of said source of aluminum. Preferably, the temperature ofsaid combination is less than or equal to 50° C. when addition of saidsource of silica begins.

Conveniently, the method further comprises allowing the synthesismixture to age at a temperature of between about 10° C. and about 30° C.for a time up to about 12 hours. In one embodiment, the synthesismixture is agitated during aging.

Conveniently, the synthesis mixture is agitated during (a) and/or (b).

Conveniently, said silicoaluminophosphate molecular sieve comprises atleast one of an AEI framework type molecular sieve and a CHA frameworktype molecular sieve.

In one embodiment, said silicoaluminophosphate molecular sieve comprisesat least one intergrown phase of an AEI framework type molecular sieveand a CHA framework type molecular sieve. Conveniently, said at leastone intergrown form has an AEI/CHA ratio of from about 5/95 to about40/60, for example from about 10/90 to about 30/70, such as from about15/85 to about 20/80, as determined by DIFFaX analysis.

In a further embodiment, the first silicoaluminophosphate molecularsieve comprises first and second intergrown forms each of an AEIframework type material and a CHA framework type material, the firstintergrown form having an AEI/CHA ratio of from about 5/95 to about40/60 as determined by DIFFaX analysis, and the second intergrown formhaving a different AEI/CHA ratio from said first intergrown form, suchas an AEI/CHA ratio of about 50/50 as determined by DIFFaX analysis.

In a further aspect, the invention resides in a silicoaluminophosphatemolecular sieve synthesized by a method described herein and its use inthe conversion of an oxygenate-containing feedstock to a productcomprising olefins.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are DIFFaX simulated diffraction patterns forintergrown AEI/CHA phases having varying AEI/CHA ratios.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is directed to a method of synthesizingsilicoaluminophosphate molecular sieves and, in particular,silicoaluminophosphate molecular sieves useful in the conversion of anoxygenate-containing feedstock, such as methanol, to a productcomprising olefins, such as ethylene and propylene.

Molecular Sieves

Crystalline molecular sieves have a 3-dimensional, four-connectedframework structure of corner-sharing [TO₄] tetrahedra, where T is anytetrahedrally coordinated cation. In the case of silicoaluminophosphates(SAPOs), the framework structure is composed of [SiO₄], [AlO₄] and [PO₄]corner sharing tetrahedral units.

Molecular sieves have been classified by the Structure Commission of theInternational Zeolite Association according to the rules of the IUPACCommission on Zeolite Nomenclature. According to this classification,framework-type zeolite and zeolite-type molecular sieves, for which astructure has been established, are assigned a three letter code and aredescribed in the Atlas of Zeolite Framework Types, 5th edition,Elsevier, London, England (2001), which is herein fully incorporated byreference.

Non-limiting examples of the molecular sieves for which a structure hasbeen established include the small pore molecular sieves of aframework-type selected from the group consisting of AEI, AFT, APC, ATN,ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR, EDI, ERI, GOO, KFI, LEV,LOV, LTA, MON, PAU, PHI, RHO, ROG, THO, and substituted forms thereof;the medium pore molecular sieves of a framework-type selected from thegroup consisting of AFO, AEL, EUO, HEU, FER, MEL, MFI, MTW, MTT, TON,and substituted forms thereof; and the large pore molecular sieves of aframework-type selected from the group consisting of EMT, FAU, andsubstituted forms thereof. Other molecular sieves have a framework-typeselected from the group consisting of ANA, BEA, CFI, CLO, DON, GIS, LTL,MER, MOR, MWW and SOD.

Non-limiting examples of the preferred molecular sieves, particularlyfor converting an oxygenate containing feedstock into olefin(s), includethose having a framework-type selected from the group consisting of AEL,AFY, BEA, CHA, EDI, FAU, FER, GIS, LTA, LTL, MER, MFI, MOR, MTT, MWW,TAM and TON.

Molecular sieve are typically described in terms of the size of the ringthat defines a pore, where the size is based on the number of T atoms inthe ring. Small pore molecular sieves generally have up to 8-ringstructures and an average pore size less than 5 Å, whereas medium poremolecular sieves generally have 10-ring structures and an average poresize of about 5 Å to about 6 Å. Large pore molecular sieves generallyhave at least 12-ring structures and an average pore size greater thanabout 6 Å. Other framework-type characteristics include the arrangementof rings that form a cage, and when present, the dimension of channels,and the spaces between the cages. See van Bekkum, et al., Introductionto Zeolite Science and Practice, Second Completely Revised and ExpandedEdition, Volume 137, pages 1-67, Elsevier Science, B.V., Amsterdam,Netherlands (2001).

Conveniently, the silicoaluminophosphate molecular sieve produced by themethod of the invention is a small pore material including an AEItopology or a CHA topology, such as SAPO-18 or SAPO-34. Preferably, themolecular sieve includes at least one intergrowth of an AEI frameworktype material and a CHA framework type material.

Regular silicoaluminophosphate molecular sieves, such as SAPO-18 andSAPO-34, are built from structurally invariant building units, calledPeriodic Building Units, and are periodically ordered in threedimensions. Structurally disordered structures show periodic ordering indimensions less than three, i.e. in two, one or zero dimensions. Thisphenomenon is called stacking disorder of structurally invariantPeriodic Building Units. Crystal structures built from Periodic BuildingUnits are called end-member structures if periodic ordering is achievedin all three dimensions. Disordered structures are those where thestacking sequence of the Periodic Building Units deviates from periodicordering up to statistical stacking sequences.

The intergrown silicoaluminophosphate molecular sieves described hereinare disordered planar intergrowth of end-member structures AEI and CHA.For AEI and CHA structure types, the Periodic Building Unit is a doublesix-ring layer. There are two types of layers “a” and “b”, which aretopologically identical except “b” is the mirror image of “a”. Whenlayers of the same type stack on top of one another, i.e. . . . aaa . .. or . . . bbb . . . , the framework type CHA is generated. When layers“a” and “b” alternate, e.g., . . . abab . . . , a different frameworktype, namely AEI, is generated. The intergrown molecular sievesdescribed herein comprise stackings of layers “a” and “b” containingregions of CHA framework type and regions of AEI framework type. Eachchange of CHA to AEI framework type is a stacking disorder or planarfault.

In the case of crystals with planar faults, the interpretation of X-raydiffraction patterns requires an ability to simulate the effects ofstacking disorder. DIFFaX is a computer program based on a mathematicalmodel for calculating intensities from crystals containing planar faults(see M. M. J. Tracey et al., Proceedings of the Royal Chemical Society,London, A [1991], Vol. 433, pp. 499-520). DIFFaX is the simulationprogram selected by and available from the International ZeoliteAssociation to simulate the XRD powder patterns for intergrown phases ofzeolites (see “Collection of Simulated XRD Powder Patterns for Zeolites”by M. M. J. Treacy and J. B. Higgins, 2001, Fourth Edition, published onbehalf of the Structure Commission of the International ZeoliteAssociation). It has also been used to theoretically study intergrownphases of AEI, CHA and KFI, as reported by K. P. Lillerud et al. in“Studies in Surface Science and Catalysis”, 1994, Vol. 84, pp. 543-550.

FIGS. 1 a and 1 b show the simulated diffraction patterns obtained forintergrowths of a CHA framework type molecular sieve with an AEIframework type molecular sieve having various AEI/CHA ratios. FIG. 1 ashows the diffraction patterns in the 5 to 25 (2θ) range simulated byDIFFaX for intergrown phases with AEI/CHA ratios of 0/100 (CHAend-member), 10/90 (AEI/CHA=0.11), 20/80 (AEI/CHA=0.25), 30/70(AEI/CHA=0.41), 40/60 (AEI/CHA=0.67), 50/50 (AEI/CHA=1.00) and 60/40(AEI/CHA=1.50). FIG. 1 b shows the diffraction patterns in the range of5 to 20 (2θ) simulated by DIFFaX for intergrown phases with AEI/CHAratios of 0/100 (CHA end-member), 10/90 (AEI/CHA=0.11), 20/80(AEI/CHA=0.25), 50/50 (AEI/CHA=1.0), 70/30 (AEI/CHA=2.33), 80/20(AEI/CHA=4.0), 100/0 (AEI end-member). All XRD diffraction patterns arenormalized to the highest peak of the entire set of simulated patterns,i.e. the peak at about 9.5 degrees 2θ for pure CHA (AEI/CHA ratio of0/100). Such normalization of intensity values allows a quantitativedetermination of mixtures of intergrowths.

As the ratio of AEI increases relative to CHA in the intergrown phase,one can observe a decrease in intensity of certain peaks, for example,the peak at about 2θ=25.0 and an increase in intensity of other peaks,for example the peak at about 2θ=17.05 and the shoulder at 2θ=21.2.Intergrown phases with AEI/CHA ratios of 50/50 and above (AEI/CHA≧1.0)show a broad feature centered at about 16.9 (2θ).

In a preferred embodiment, the intergrown silicoaluminophosphatemolecular sieve produced by the method of the invention is at least oneintergrowth of an AEI framework type and a CHA framework type, whereinsaid at least one intergrowth has an AEI/CHA ratio of from about 5/95 toabout 40/60, for example from about 10/90 to about 30/70, such as fromabout 15/85 to about 20/80, as determined by DIFFaX analysis. Such aCHA-rich intergrowth is characterized by a powder XRD diffractionpattern (obtained from a sample after calcination and withoutrehydration after calcination) having at least the reflections in the 5to 25 (2θ) range as shown in Table below.

TABLE 1 2θ (CuKα) 9.3-9.6 12.7-13.0 13.8-14.0 15.9-16.1 17.7-18.118.9-19.1 20.5-20.7 23.7-24.0

The X-ray diffraction data referred to herein are collected with aSCINTAG X2 X-Ray Powder Diffractometer (Scintag Inc., USA), using copperK-alpha radiation. The diffraction data are recorded by step-scanning at0.02 degrees of two-theta, where theta is the Bragg angle, and acounting time of 1 second for each step. Prior to recording of eachexperimental X-ray diffraction pattern, the sample must be in theanhydrous state and free of any template used in its synthesis, sincethe simulated patterns are calculated using only framework atoms, notextra-framework material such as water or template in the cavities.Given the sensitivity of silicoaluminophosphate materials to water atrecording temperatures, the molecular sieve samples are calcined afterpreparation and kept moisture-free according to the following procedure.

About 2 grams of each molecular sieve sample are heated in an oven fromroom temperature under a flow of nitrogen at a rate of 3° C./minute to200° C. and, while retaining the nitrogen flow, the sample is held at200° C. for 30 minutes and the temperature of the oven is then raised ata rate of 2° C./minute to 650° C. The sample is then retained at 650° C.for 8 hours, the first 5 hours being under nitrogen and the final 3hours being under air. The oven is then cooled to 200° C. at 30°C./minute and, when the XRD pattern is to be recorded, the sample istransferred from the oven directly to a sample holder and covered withMylar foil to prevent rehydration. Recording under the same conditionsimmediately after removal of the Mylar foil will also provide adiffraction pattern suitable for use in DIFFaX analysis.

In an alternative embodiment, the intergrown silicoaluminophosphatemolecular sieve produced by the method of the invention comprises aplurality of intergrown forms of the CHA and AEI framework types,typically with a first intergrown form having an AEI/CHA ratio of fromabout 5/95 to about 40/60, as determined by DIFFaX analysis, and asecond intergrown form having a different AEI/CHA ratio from said firstintergrown form. The second intergrown form typically has an AEI/CHAratio of about 30/70 to about 55/45, such as about 50/50, as determinedby DIFFaX analysis, in which case the XRD diffraction pattern exhibits abroad feature centered at about 16.9 (2θ) in addition to the reflectionpeaks listed in Table 1.

Preferably, the CHA framework type molecular sieve in the AEI/CHAintergrowths described above is SAPO-34 and the AEI framework typemolecular sieve is selected from SAPO-18, ALPO-18 and mixtures thereof.In addition, the intergrown silicoaluminophosphate preferably has aframework silica to alumina molar ratio (Si/Al₂) greater than 0.16 andless than 0.19, such as from about 0.165 to about 0.185, for exampleabout 0.18. The framework silica to alumina molar ratio is convenientlydetermined by NMR analysis.

Molecular Sieve Synthesis

Generally, silicoaluminophosphate molecular sieves are synthesized bythe hydrothermal crystallization of a source of aluminum, a source ofphosphorus, a source of silicon and at least one organic directingagent. Typically, a combination of sources of silicon, aluminum andphosphorus, together with one or more directing agents and optionallyseeds from another or the same framework type molecular sieve, is placedin a sealed pressure vessel, optionally lined with an inert plastic suchas polytetrafluoroethylene, and heated, under a crystallization pressureand temperature, until a crystalline material is formed, and thenrecovered by filtration, centrifugation and/or decanting.

Non-limiting examples of suitable silicon sources include silicates,fumed silica, for example, Aerosil-200 available from Degussa Inc., NewYork, N.Y., and CAB-O-SIL M-5, organosilicon compounds such astetraalkyl orthosilicates, for example, tetramethyl orthosilicate (TMOS)and tetraethylorthosilicate (TEOS), colloidal silicas or aqueoussuspensions thereof, for example Ludox HS-40 sol available from E.I. duPont de Nemours, Wilmington, Del., silicic acid or any combinationthereof.

Non-limiting examples of suitable aluminum sources includeorganoaluminum compounds such as aluminum alkoxides, for examplealuminum isopropoxide, and inorganic aluminum sources, such as aluminumphosphate, aluminum hydroxide, sodium aluminate, pseudo-boehmite,gibbsite and aluminum trichloride, or any combination thereof. Preferredsources are inorganic aluminum compounds, such as hydrated aluminumoxides and particularly boehmite and pseudoboehmite.

Non-limiting examples of suitable phosphorus sources, which may alsoinclude aluminum-containing phosphorus compositions, include phosphoricacid, organic phosphates such as triethyl phosphate, and crystalline oramorphous aluminophosphates such as AlPO₄, phosphorus salts, orcombinations thereof. A preferred source of phosphorus is phosphoricacid.

The organic directing agents employed in the synthesis ofsilicoaluminophosphate molecular sieves generally contain at least oneelement of Group 15 of the Periodic Table of Elements and at least onealkyl or aryl group, such as an alkyl or aryl group having from 1 to 10carbon atoms, for example from 1 to 8 carbon atoms. Preferred directingagents are basic nitrogen-containing compounds, such as amines,quaternary ammonium compounds and combinations thereof. Suitablequaternary ammonium compounds are represented by the general formulaR₄N⁺, where each R is hydrogen or a hydrocarbyl or substitutedhydrocarbyl group, preferably an alkyl group or an aryl group havingfrom 1 to 10 carbon atoms.

Non-limiting examples of suitable directing agents include tetraalkylammonium compounds including salts thereof, such as tetramethyl ammoniumcompounds, tetraethyl ammonium compounds, tetrapropyl ammoniumcompounds, and tetrabutylammonium compounds, cyclohexylamine,morpholine, di-n-propylamine (DPA), tripropylamine, triethylamine (TEA),triethanolamine, piperidine, cyclohexylamine, 2-methylpyridine,N,N-dimethylbenzylamine, N,N-diethylethanolamine, dicyclohexylamine,N,N-dimethylethanolamine, choline, N,N′-dimethylpiperazine,1,4-diazabicyclo(2,2,2)octane, N′,N′,N,N-tetramethyl-(1,6)hexanediamine,N-methyldiethanolamine, N-methyl-ethanolamine, N-methyl piperidine,3-methyl-piperidine, N-methylcyclohexylamine, 3-methylpyridine,4-methyl-pyridine, quinuclidine, N,N′-dimethyl-1,4-diazabicyclo(2,2,2)octane ion; di-n-butylamine, neopentylamine, di-n-pentylamine,isopropylamine, t-butyl-amine, ethylenediamine, pyrrolidine, and2-imidazolidone.

Since the organic directing agent used in the synthesis ofsilicoaluminophosphates is typically a basic nitrogen compound, whereasan attractive phosphorus source is phosphoric acid, it will beappreciated that the mixing of these materials can generate heat andhence raise the temperature of the synthesis mixture. According to theinvention, it has been found that such a rise in temperature can lead toundesirable side reactions and possible production of impurity phases ifthe aluminum source is added to the synthesis mixture, and moreparticularly if both the aluminum and silicon sources are added to thesynthesis mixture, when the temperature of the mixture exceeds 50° C.Generally, it is found that the temperature of the combination of thephosphorus source and organic directing agent should be less than orequal to 40° C., preferably less than or equal to 30° C., and morepreferably in the range of about 110° C. to about 30° C., when additionof the aluminum source begins and preferably when addition of thesilicon source begins.

To alleviate the above-mentioned problem, the temperature of thesynthesis mixture is preferably controlled during the initialcombination of the source of phosphorus with the at least one organicdirecting agent so that the temperature of the mixture is less than orequal to 50° C., preferably less than or equal to 40° C., morepreferably less than or equal to 30° C., and most preferably in therange of about 10° C. to about 30° C., during substantially the entirecombination process. Generally, this can be achieved by effecting one ormore of controlling the rate of addition of the phosphorus source andorganic directing agent to the mixture, pre-cooling one or both of thephosphorus source and organic directing agent; and cooling the mixtureduring the mixing process. With large commercial-scale syntheses, inwhich the total weight of the synthesis mixture exceeds 1 kg, or even 5kg, it may be difficult to maintain close control over the temperatureduring the entire process of mixing the phosphorus source and organicdirecting agent, but short term temperature spikes above 50° C. arenormally acceptable provided the temperature returns to less than orequal to 50° C. before the aluminum source addition begins.

The silicon source can be introduced into the combination of thephosphorus source and organic directing agent before, after orsimultaneously with the aluminum source, but is normally added when thetemperature of the mixture is less than or equal to 50° C.

After combining all the components of the synthesis mixture, the mixtureis desirably allowed to age at a temperature of between about 10° C. andabout 30° C. for a time up to about 12 hours. Conveniently, thesynthesis mixture is agitated during the aging step.

After the optional aging step, the synthesis mixture is sealed in avessel and heated, preferably under autogenous pressure, to atemperature in the range of from about 100° C. to about 300° C., forexample from about 125° C. to about 250° C., such as from about 150° C.to about 200° C. The time required to form the crystalline product isusually dependent on the temperature and can vary from immediately up toseveral weeks. Typically the crystallization time is from about 30minutes to about 2 weeks, such as from about 45 minutes to about 240hours, for example from about 1 hour to about 120 hours. Thehydrothermal crystallization may be carried out without or, morepreferably, with agitation.

Once the crystalline molecular sieve product is formed, usually in aslurry state, it may be recovered by any standard technique well knownin the art, for example, by centrifugation or filtration. The recoveredcrystalline product may then be washed, such as with water, and thendried, such as in air.

In one practical embodiment, wherein the silicoaluminophosphatemolecular sieve comprises a CHA/AEI intergrowth as described above, thesynthesis method comprises:

a) combining sources of silicon, phosphorus and aluminum with an organicstructure directing agent (template) in the manner described above toform a mixture having a molar composition within the following ranges:

P₂O₅:Al₂O₃ from about 0.6 to about 1.2,

SiO₂:Al₂O₃ from about 0.005 to about 0.35,

H₂O:Al₂O₃ from about 10 to about 50;

b) mixing and heating the mixture (a) continuously to a crystallizationtemperature, such as between about 100° C. and about 250° C., typicallybetween about 140° C. and about 180° C., preferably between about 150°C. and about 170° C.;

c) maintaining the mixture at the crystallization for a period of timeof from about 2 to about 150 hours; such as from about 5 to about 100hours, for example from about 10 to about 50 hours; and

(d) recovering a crystalline product containing the desired molecularsieve.

As a result of the synthesis process, the crystalline product recoveredsuch as in step (d) above contains within its pores at least a portionof the organic directing agent used in the synthesis. In a preferredembodiment, activation is performed in such a manner that the organicdirecting agent is removed from the molecular sieve, leaving activecatalytic sites within the microporous channels of the molecular sieveopen for contact with a feedstock. The activation process is typicallyaccomplished by calcining, or essentially heating the molecular sievecomprising the template at a temperature of from about 200° C. to about800° C. in the presence of an oxygen-containing gas. In some cases, itmay be desirable to heat the molecular sieve in an environment having alow or zero oxygen concentration. This type of process can be used forpartial or complete removal of the organic directing agent from theintracrystalline pore system.

Molecular Sieve Catalyst Compositions

The silicoaluminophosphate molecular sieves produced by the synthesismethod of the invention are particularly intended for use as organicconversion catalysts. Before use in catalysis, the molecular sieves willnormally be formulated into catalyst compositions by combination withother materials, such as binders and/or matrix materials, which provideadditional hardness or catalytic activity to the finished catalyst.

Materials which can be blended with the molecular sieve can be variousinert or catalytically active materials. These materials includecompositions such as kaolin and other clays, various forms of rare earthmetals, other non-zeolite catalyst components, zeolite catalystcomponents, alumina or alumina sol, titania, zirconia, quartz, silica orsilica sol, and mixtures thereof. These components are also effective inreducing overall catalyst cost, acting as a thermal sink to assist inheat shielding the catalyst during regeneration, densifying the catalystand increasing catalyst strength. When blended with such components, theamount of molecular sieve contained in the final catalyst product rangesfrom about 10 to about 90 weight percent of the total catalyst,preferably about 20 to about 80 weight percent of the total catalystcomposition.

Use of the Molecular Sieve

The silicoaluminophosphate molecular sieves produced by the method ofthe invention are useful as catalysts in a variety of processesincluding cracking of, for example, a naphtha feed to light olefin(s) orhigher molecular weight (MW) hydrocarbons to lower MW hydrocarbons;hydrocracking of, for example, heavy petroleum and/or cyclic feedstock;isomerization of, for example, aromatics such as xylene; polymerizationof, for example, one or more olefin(s) to produce a polymer product;reforming; hydrogenation; dehydrogenation; dewaxing of, for example,hydrocarbons to remove straight chain paraffins; absorption of, forexample, alkyl aromatic compounds for separating out isomers thereof;alkylation of, for example, aromatic hydrocarbons such as benzene andalkyl benzene, optionally with propylene to produce cumene or with longchain olefins; transalkylation of, for example, a combination ofaromatic and polyalkylaromatic hydrocarbons; dealkylation;dehydrocyclization; disproportionation of, for example, toluene to makebenzene and paraxylene; oligomerization of, for example, straight andbranched chain olefin(s); and dehydrocyclization.

Where the silicoaluminophosphate is an AEI or CHA structure typematerial or an intergrowth of an AEI structure type material and a CHAstructure type material, the molecular sieve produced by the method ofthe invention is particularly suitable as a catalyst for use in theconversion of oxygenates to olefins. As used herein, the term“oxygenates” is defined to include, but is not necessarily limited toaliphatic alcohols, ethers, carbonyl compounds (aldehydes, ketones,carboxylic acids, carbonates, and the like), and also compoundscontaining hetero-atoms, such as, halides, mercaptans, sulfides, amines,and mixtures thereof. The aliphatic moiety will normally contain fromabout 1 to about 10 carbon atoms, such as from about 1 to about 4 carbonatoms.

Representative oxygenates include lower straight chain or branchedaliphatic alcohols, their unsaturated counterparts, and their nitrogen,halogen and sulfur analogues. Examples of suitable oxygenate compoundsinclude methanol; ethanol; n-propanol; isopropanol; C₄-C₁₀ alcohols;methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether;methyl mercaptan; methyl sulfide; methyl amine; ethyl mercaptan;di-ethyl sulfide; di-ethyl amine; ethyl chloride; formaldehyde;di-methyl carbonate; di-methyl ketone; acetic acid; n-alkyl amines,n-alkyl halides, n-alkyl sulfides having n-alkyl groups of comprisingthe range of from 3 to 10 carbon atoms; and mixtures thereof.Particularly suitable oxygenate compounds are methanol, dimethyl ether,or mixtures thereof, most preferably methanol. As used herein, the term“oxygenate” designates only the organic material used as the feed. Thetotal charge of feed to the reaction zone may contain additionalcompounds, such as diluents.

In the present oxygenate conversion process, a feedstock comprising anorganic oxygenate, optionally with one or more diluents, is contacted inthe vapor phase in a reaction zone with a catalyst comprising themolecular sieve of the present invention at effective process conditionsso as to produce the desired olefins. Alternatively, the process may becarried out in a liquid or a mixed vapor/liquid phase. When the processis carried out in the liquid phase or a mixed vapor/liquid phase,different conversion rates and selectivities of feedstock-to-product mayresult depending upon the catalyst and the reaction conditions.

When present, the diluent(s) is generally non-reactive to the feedstockor molecular sieve catalyst composition and is typically used to reducethe concentration of the oxygenate in the feedstock. Non-limitingexamples of suitable diluents include helium, argon, nitrogen, carbonmonoxide, carbon dioxide, water, essentially non-reactive paraffins(especially alkanes such as methane, ethane, and propane), essentiallynon-reactive aromatic compounds, and mixtures thereof. The mostpreferred diluents are water and nitrogen, with water being particularlypreferred. Diluent(s) may comprise from about 1 mol % to about 99 mol %of the total feed mixture.

The temperature employed in the oxygenate conversion process may varyover a wide range, such as from about 200° C. to about 1000° C., forexample from about 250° C. to about 800° C., including from about 250°C. to about 750° C., conveniently from about 300° C. to about 650° C.,typically from about 350° C. to about 600° C. and particularly fromabout 400° C. to about 600° C.

Light olefin products will form, although not necessarily in optimumamounts, at a wide range of pressures, including but not limited toautogenous pressures and pressures in the range of from about 0.1 kPa toabout 10 MPa. Conveniently, the pressure is in the range of from about 7kPa to about 5 MPa, such as in the range of from about 50 kPa to about 1MPa. The foregoing pressures are exclusive of diluent, if any ispresent, and refer to the partial pressure of the feedstock as itrelates to oxygenate compounds and/or mixtures thereof. Lower and upperextremes of pressure may adversely affect selectivity, conversion,coking rate, and/or reaction rate; however, light olefins such asethylene still may form.

The process should be continued for a period of time sufficient toproduce the desired olefin products. The reaction time may vary fromtenths of seconds to a number of hours. The reaction time is largelydetermined by the reaction temperature, the pressure, the catalystselected, the weight hourly space velocity, the phase (liquid or vapor)and the selected process design characteristics.

A wide range of weight hourly space velocities (WHSV) for the feedstockwill function in the present process. WHSV is defined as weight of feed(excluding diluent) per hour per weight of a total reaction volume ofmolecular sieve catalyst (excluding inerts and/or fillers). The WHSVgenerally should be in the range of from about 0.01 hr⁻¹ to about 500hr⁻¹, such as in the range of from about 0.05 hr⁻¹ to about 300 hr⁻¹,for example in the range of from about 0.1 hr⁻¹ to about 200 hr⁻¹.

A practical embodiment of a reactor system for the oxygenate conversionprocess is a circulating fluid bed reactor with continuous regeneration,similar to a modern fluid catalytic cracker. Fixed beds are generallynot preferred for the process because oxygenate to olefin conversion isa highly exothermic process which requires several stages withintercoolers or other cooling devices. The reaction also results in ahigh pressure drop due to the production of low pressure, low densitygas.

Because the catalyst must be regenerated frequently, the reactor shouldallow easy removal of a portion of the catalyst to a regenerator, wherethe catalyst is subjected to a regeneration medium, such as a gascomprising oxygen, for example air, to burn off coke from the catalyst,which restores the catalyst activity. The conditions of temperature,oxygen partial pressure, and residence time in the regenerator should beselected to achieve a coke content on regenerated catalyst of less thanabout 0.5 wt %. At least a portion of the regenerated catalyst should bereturned to the reactor.

Using the various oxygenate feedstocks discussed above, particularly afeedstock containing methanol, a catalyst composition of the inventionis effective to convert the feedstock primarily into one or moreolefin(s). The olefin(s) produced typically have from 2 to 30 carbonatoms, preferably 2 to 8 carbon atoms, more preferably 2 to 6 carbonatoms, still more preferably 2 to 4 carbons atoms, and most preferablyare ethylene and/or propylene. The resultant olefins can be separatedfrom the oxygenate conversion product for sale or can be fed to adownstream process for converting the olefins to, for example, polymers.

The invention will now be more particularly described with reference tothe following Examples.

In the Examples, DIFFaX analysis was used to determine the AEI/CHA ratioof the molecular sieves. Simulated powder XRD diffraction patterns forvarying ratios of AEI/CHA were generated using the DIFFaX programavailable from the International Zeolite Association (see also M. M. J.Tracey et al., Proceedings of the Royal Chemical Society, London, A(1991), Vol. 433, pp. 499-520 “Collection of Simulated XRD PowderPatterns for Zeolites” by M. M. J. Treacy and J. B. Higgins, 2001,Fourth Edition, published on behalf of the Structure Commission of theInternational Zeolite Association). The DIFFaX input file used tosimulate the XRD diffraction patterns is given in Table 2 of U.S. PatentApplication Publication No. 2002/0165089, incorporated herein byreference. In order to obtain best fitting between the DIFFaX simulatedpatterns and the experimental patterns, two sets of simulated XRDpatterns were generated using a line broadening of 0.009 (as describedin U.S. Patent Application No. 2002/0165089) and a line broadening of0.04 (FIGS. 1 a and 1 b). The simulated diffraction patterns were thencompared with the experimental powder XRD diffraction patterns. In thisrespect, a very sensitive range is the 15 to 19.5 2θ range.

Example 1

A mixture of 5017 kg of phosphoric acid (85% in water), 4064 kg ofdemineralized water and 9157 kg of tetraethylammonium hydroxide solution(35% in water, Sachem) was prepared. After initiating stirring of themixture and cooling the mixture to 30° C., 392 kg Ludox AS 40 (40%silica) was added to the beaker followed by 2873 kg of alumina (CondeaPural SB-1) and 422 kg of rinse water. The composition of the finalsynthesis mixture in terms of molar ratios was as follows:0.12SiO₂/Al₂O₃/P₂O₅/TEAOH/34H₂O

The mixture was transferred to a stainless steel autoclave and heated ata rate of 20° C./hour to 165° C. while being stirred with a mixer at aspeed of 31 rpm. The autoclave was kept at 165° C. for 72 hours. Aftercooling to room temperature, the slurry was washed. The washed slurryhad a d₅₀ particle size of 1.7 μm, as measured with a MalvernMastersizer 2000 (d₅₀ expressed by volume). The washed slurry was thendried and an X-ray diffraction pattern of the crystalline product wastaken after the calcination procedure described above. Using thisdiffraction pattern, DIFFaX analysis was conducted and showed thecrystalline product to contain an AEI/CHA intergrowth comprising 26 wt %of an AEI structure type molecular sieve and 74 wt % of a CHA structuretype molecular sieve. The silica to alumina molar ratio (Si/Al₂) of thecrystalline product was found to be 0.16 and the yield of the AEI/CHAintergrowth was 20.2 wt % of the solids in the starting mixture.

Examples 2 to 6

The procedure of Example 1 was repeated but with the mixture ofphosphoric acid, demineralized water and tetraethylammonium hydroxidesolution being cooled to various temperatures between 10° C. and 50° C.before addition of the silica and alumina. Apart from some minor changesin the rate of heating to the crystallization temperature of 165° C.,all the other parameters of Example 1 remained the same. The results aresummarized in Table 2, where “Other” designates any impurity phasespresent in the crystalline product as determined by the X-ray analysisand “Cooling Temp” indicates the temperature to which the mixture ofphosphoric acid, demineralized water and tetraethylammonium hydroxidesolution was cooled before addition of the silica and alumina.

TABLE 2 Cooling Heat Rate Yield Al₂/Si Ex Temp. ° C. ° C./hr wt %AEI/CHA Other Product 2 10 23 20.0 25/75 AFI 0.16 3 15 20 20.4 24/760.15 4 20 21 19.7 27/73 0.15 5 40 20 20.1 26/74 APC 0.16 6 50 20 22.530/70 0.14

Examples 7 and 8

The procedure of Example 1 was repeated but with the crystallizationtime at 165° C. being either 48 hours (Example 7) or 60 hours (Example8), all the other parameters of Example 1 remaining the same. As inExample 1, the mixture of phosphoric acid, demineralized water andtetraethylammonium hydroxide solution was cooled to 30° C. beforeaddition of the silica and alumina. The results are summarized in Table3, where “Other” designates any impurity phases present in thecrystalline product as determined by X-ray analysis.

TABLE 3 Heat Rate Yield Al₂/Si Ex ° C./hr wt % AEI/CHA Other Product 720 19.5 30/70 APC 0.15 8 20 20.2 27/73 0.16

Example 9 (Comparative)

8221.1 g of phosphoric acid (85% in water) was diluted with 6693.8 .g ofdemineralized water. To this solution was added 535.6 g of Ludox AS40(40% silica in water) and 147.1 g of water. After mixing untilhomogeneous, 15001.3 g of tetraethylammonium hydroxide solution (35% inwater, Sachem) and 147.1 g of demineralized water were added and themixture was stirred until homogeneous. Then 4886.2 g of alumina (CondeaPural SB-1) and 367.8 g of rinse water were added. The temperature ofthe mixture was 53° C. The composition of the final synthesis mixture interms of molar ratios was as follows:0.10SiO₂/Al₂O₃/P₂O₅/TEAOH/34H₂O

The mixture was transferred to a stainless steel autoclave and heated in12 hrs to 175° C. while being stirred with a mixer at a speed of 49 rpm.The autoclave was kept at 175° C. for 17 hours. After cooling to roomtemperature, the slurry was washed. The washed slurry was then dried andan X-ray diffraction pattern of the crystalline product was taken. Theproduct was identified to be a mixture of SAPO-5 and SAPO-34.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations notnecessarily illustrated herein. For this reason, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

1. A method of synthesizing a silicoaluminophosphate molecular sieve,the method consisting of: (a) preparing a synthesis mixture comprisingthe steps of: (i) combining a source of phosphorus and at least oneorganic directing agent, wherein the mixing of said phosphorus sourceand said organic directing agent can generate heat and raise thetemperature of said synthesis mixture; but wherein the temperature ofthe combination of said phosphorus source and said organic directingagent is controlled so that said temperature does not exceed 50° C.during substantially this entire step (i); (ii) introducing a source ofaluminum into the combination of the phosphorus source and organicdirecting agent, the temperature of said combination being less than orequal to 50° C. when addition of said source of aluminum begins; and(iii) introducing a source of silicon into said combination of thephosphorus source and the organic directing agent after orsimultaneously with the introduction of said source of aluminum, thetemperature of said combination being less than or equal to 50° C. whenaddition of said source of silicon begins, and (iv) further comprisingallowing said synthesis mixture to age at a temperature of between 10°C. and 30° C. for a time up to about 12 hours; (b) heating saidsynthesis mixture to a crystallization temperature of between about 100°C. and about 300° C.; and (c) recovering a SAPO-18, SAPO-34, or mixturesthereof molecular sieve.
 2. The method of claim 1, wherein thetemperature of said combination of the phosphorus source and organicdirecting agent is less than or equal to 40° C., when addition of saidsource of aluminum begins.
 3. The method of claim 1, wherein thetemperature of said combination of the phosphorus source and organicdirecting agent is in the range of about 10° C. to about 30° C. whenaddition of said source of aluminum begins.
 4. The method of claim 1,wherein the temperature of said combination is controlled by cooling thecombination and/or cooling one or both of the phosphorus source andorganic directing agent.
 5. The method of claim 1, wherein said sourceof aluminum is an inorganic aluminum compound.
 6. The method of claim 1,wherein said source of aluminum is selected from a hydrated aluminumoxide, boehmite and pseudoboehmite.
 7. The method of claim 1, whereinthe source of silicon is introduced into said combination of thephosphorus source and organic directing agent after introduction of saidsource of aluminum.
 8. The method of claim 1, wherein the source ofsilicon is introduced into said combination of the phosphorus source andorganic directing agent simultaneously with the introduction of saidsource of aluminum.
 9. The method of claim 1, wherein said source ofphosphorus is phosphoric acid.
 10. The method of claim 1, wherein saidorganic directing agent is a nitrogen compound.
 11. The method of claim1 and further comprising agitating said synthesis mixture during (a)and/or (b).
 12. The method of claim 1, wherein saidsilicoaluminophosphate molecular sieve comprises at least one of an AEIstructure type molecular sieve and a CHA structure type molecular sieve.13. The method of claim 1, wherein said silicoaluminophosphate molecularsieve comprises at least one intergrown phase of an AEI structure typemolecular sieve and a CHA structure type molecular sieve.
 14. The methodof claim 13, wherein said at least one intergrown phase has an AEI/CHAratio of from 5/95 to 40/60, as determined by DIFFaX analysis.
 15. Themethod of claim 13, wherein said silicoaluminophosphate molecular sievehas an X-ray diffraction pattern comprising at least one reflection peakin each of the following ranges in the 5 to 25 (2θ) range: 2θ (CuKα)9.3-9.6 12.7-13.0 13.8-14.0 15.9-16.1 17.7-18.1 18.9-19.1 20.5-20.7 23.7-24.0.


16. The method of claim 1, wherein said silicoaluminophosphate molecularsieve comprises first and second intergrown forms each of an AEIframework type material and a CHA framework type material.
 17. Themethod of claim 16, wherein said first intergrown form has an AEI/CHAratio of from about 5/95 to about 40/60 as determined by DIFFaX analysisand said second intergrown form has a different AEI/CHA ratio from saidfirst intergrown form.
 18. The method of claim 17, wherein said secondintergrown form has an AEI/CHA ratio of about 30/70 to about 55/45, asdetermined by DIFFaX analysis.